NO150499B - ROTATING ELECTRICAL CHARGING PRINTER - Google Patents

Abstract

Rotating electric discharge printer.

Description

Method and apparatus for the production of cellulose.

The present invention relates to the production of cellulose pulp by continuous boiling of wood or other cellulose-containing raw materials.

Paper pulp has been commercially produced by immersing the chip in an excess of caustic soda and using this excess of lye which can be circulated through the chip pulp as a heat exchange medium. For example, steam can be fed to a zone near the bottom of the boiler and the lye thus heated is circulated to heat the tile. Alternatively, the lye can be circulated by means of a pump through a heat exchanger located outside the boiler and then returned to the boiler.

In normal boiling, heat and pressure are released for the cooker to remove non-condensable gas. Also at the end of boiling, the pressure is released over a short time. The steam emitted during the release of the non-condensable gases and at the end of boiling can be condensed and recovered as hot water.

It has been known for some time from laboratory experiments that boiling can be carried out using vapor phase heating. With this technique, the tile is flooded with caustic soda of a specific concentration and, after the tile has been sufficiently impregnated, the excess liquid is removed. The initial concentration for the lye is regulated so that sufficient chemicals will be retained in the tile to complete boiling after removal of the excess of lye. The actual boiling is then carried out by heating the impregnated chips in a pressure vessel to a certain temperature at which they are kept until the boiling is complete and at which point the contents of the boiler are cooled and the cellulose mass formed is separated from the residual liquor.

When attempts have been made to produce chemical pulp using the above method on a commercial scale, it has been found that it is impossible to maintain a correct water balance. This is due to the fact that the volume of the water contained in the cooking liquor together with the tile moisture and the condensate from the steam is normally greater than the volume of the liquid that the tile will absorb.

With the help of the present invention, the problem of water balance is now solved. In addition, devices have now been found to efficiently combine the boiling step with the washing and lye evaporation step so that the overall effect of the process is improved.

The main purpose of the present invention is thus to provide an improved system for maintaining a correct water balance in the material being boiled.

The impregnation system, which is one of the main features of the present invention, comprises a simultaneous evaporation of water to maintain such a ratio between lye and wood that there will be no excess of what is required to completely impregnate the tile, as the ratio is accurate and easily maintained despite variations in moisture content in the input, chips, lye, etc.

A further purpose is to provide an improved system for impregnation, boiling, washing and evaporation.

Another feature of the present invention is the economy in terms of chemical requirements, as less chemicals are required in the system to impregnate the tile.

Another purpose is to provide a system that has a better total heat economy.

A substantial part of the residual heat from this combined impregnation, boiling, washing and evaporation system exists in the form of steam which can be used for other purposes. This is in contrast to the lower temperature steam and lye available from batch operation. In addition, it is possible to reduce the total heat exchange surface. Furthermore, a high percentage of secondary heat from the system can be recovered in the form of hot water.

A further purpose of the invention is an improved washing system which allows a higher black liquor concentration at an elevated temperature from the washing system.

Another feature of the present system is the condensate recovery from which sufficient water is available to meet all needs from the laundry together with part of the water required in the black liquor recovery system.

Other purposes and advantages will be apparent from the following detailed description in connection with the drawings. Fig. 1 schematically shows an embodiment of the system. Fig. 2 shows , a system where lye is evaporated outside the impregnator. Fig. 3 schematically shows an alternative type of washing system.

Chips fed from the chip bin 10 are first subjected to a basin in a container 11 and then pass through a rotary valve 12 into an impregnation pipe 13 which is partially filled with lye. Inside the pipe 13, the tile is impregnated at regulated pressure, temperature, lye concentration and time. The impregnated wood chip which is free of excess ends is then discharged through another rotary valve 14 to the cooking pipe 15 where the wood chip is heated by means of direct steam through the steam pipe 16 and the cooking step is carried out at a regulated temperature and time. As the boiling progresses during passage through the boiler 15, the tile shrinks and black liquor is drained from it and the exhausted black liquor is removed from the boiler tube 15 through the pipe 32.

From the cooking vessel, the tile passes through a third rotary valve 18 to the pipe 19 where it is led by means of a screw conveyor 20 where a major part of the heat is removed with the lye and the tile is partially washed by countercurrent contact - with washing water from a further step such as a pressure washer 21. The washer 21 provides another washing step in the washing system. The boiled and washed cellulose mass is extracted with a high consistency through the tube 22.

Impregnation and water balance.

With the help of the present system for boiling, all the chemicals required for boiling the tile are present in situ in the impregnated tile at the time they are brought to the boiling temperature. The boiling rate is thus only regulated by the chemical reaction rates and the boiling time can therefore be relatively short compared to normal systems where a significant part of the chemical is outside the tile and the boiling rate is regulated by diffusion. This shortened boiling time with the present system reduces unwanted side reactions that occur in the lye and this, together with the absence of recycled black liquor, reduces the amount of chemicals required. The reduced lye-to-wood ratio with the present system also results in a residual lye that has a higher concentration and thus reduces the evaporation required.

Attempts to produce chemical cellulose pulp on a commercial scale using vapor phase cooking have been hindered by the fact that normally more water is added to the system (through the content of the chip, water in the cooking liquor and the condensate from evaporation of the chip) than the chip will carry from the impregnation. It has now been found that a satisfactory water balance can be maintained by carrying out a varying evaporation in the impregnation container, as the evaporation depends on the moisture content in the tile and the lye concentration. This control is achieved according to the present invention by superheating the recirculating liquor to a few degrees higher than the temperature maintained in the impregnation vessel and allowing the water to evaporate either at the end of the vessel or in a separate evaporation tank somewhere after the heating and before re-tour to the cooker. By-product steam at approximately 1.0-5.2 kg/cm<2> pressure depending on the temperature in the pipe is thus available for further use.

Many forms of impregnation container can be used, one type being illustrated schematically in fig. 1. This impregnation consists of a cylindrical impregnation container 13 placed at an angle in relation to the horizontal and provided with a central body which divides the pipe into two parts. The angle of this pipe is usually about 45° with the horizontal, but this angle can be varied within certain limits (about 30° to 60° with respect to the horizontal). The chip is fed into the impregnation tube 13 by means of a type of conveyor with regulated low speed or the like, schematically indicated at 24 and is moved downwards on the upper side of the middle partition 23 and upwards along the lower side of the middle partition.

The lye is circulated through the impregnation container 13 via the pipe 25 to a heat exchanger 26 and back to the top of the container 13 through the pipe 29. The heat exchanger is heated using high-pressure steam from the steam pipe 27. To maintain the concentration in circulated lye, supplementary lye is put through the pipe 28 and the now reinforced lye is heated in the heat exchanger 26 and recycled through the pipe 29 to the impregnation pipe 13 where it is held at a certain pressure by means of the pressure control valve 30' in the pipe 30. This reinforced lye is heated to a certain temperature which is higher than the equilibrium temperature which corresponds to the impregnation pipe's pressure passes through the tube 29 through the top of the container 13 above the level of the lye in this and part evaporates into steam. The steam produced in this way is discharged at a specific pressure through the pipe 30.

Impregnated chips are fed by the conveyor 24 to a point above the lye level in the impregnation pipe 13 and discharged through a rotating valve 14.

A relatively constant level of liquid is maintained in the impregnation pipe, which is kept at the specified pressure by regulating the temperature in the recirculation end. If the liquid level in the pipe 13 should fall below a certain point, less steam is supplied to the heat exchanger 26 with the resulting lowering of the temperature of the recirculating lye, which in turn reduces the amount of steam formed during evaporation. This means that a greater proportion of water remains in the lye and consequently the level of the lye in the container 13 tends to rise. Conversely, if the level were to rise above a certain point, more heat is supplied to the heat exchanger 26 to increase the temperature in the recirculation end and thus greater evaporation is caused when the lye is evaporated to thereby lower the level of the lye. The level regulation described above can be operated automatically by means of suitable devices which react to the level L and regulate the heat supply to the heat exchanger 26 while the pressure is kept constant.

The impregnation liquid can be adjusted to

to maintain the correct chemical concentration either manually or if suitable equipment is available automatically by regulating the amount of supplementary lye added to the tube 28 to give the desired concentration. The concentration of active cooking ingredient in the lye is thus regulated to a specific value so that the tile leaving the impregnation will have the correct amount of chemicals for cooking in the following step.

It should be noted that the conveyor speed is variable and thus the feeding time through the impregnation pipe can be varied as desired. The temperature maintained in this tube can be any suitable impregnation temperature.

It should also be noted that the pressure for the treatment steam that is removed via pipe 30 corresponds to the steam pressure in the lye held in the impregnation pipe. The temperature of this lye can also be regulated to the correct impregnation temperature for the tile.

Another impregnation system incorporated in a different type of boiler than that shown in fig. 1 is shown in fig. 2. In the system in fig. 2 evaporation occurs outside the impregnator. As shown, the evaporation system is connected to an updraft boiler 100 with impregnation in the upper zone.

The lye is removed from the impregnation zone through pipe 25A and then passes through a heat exchanger 26A where it is heated by the steam from pipe 27a. Fresh chemicals

is added to the system via pipe 28a. The amount of heat used in the heat exchanger

26a is regulated according to the level L' in the boiler 100

in the same way as the heat exchanger 26

is regulated according to the level L as described above.

Heated and reinforced lye is fed via pipe 29A into an evaporation tank 13a.

The evaporation tank is kept at a certain pressure and temperature, as is the impregnator 13, so that water will evaporate in the tank 13a. The amount of water that is evaporated depends on the difference in temperature between the heated reinforced lye in the tube 29a and the lye in the evaporation tank. Steam generated by evaporation of lye in tank 13a is removed via pipe 30b which is connected to evaporator 44 like pipe 30 in the first embodiment. A pressure regulating valve 30C regulates the pressure in the evaporation tank 13a.

After evaporation in tank 13a, lye is returned to the impregnation zone via pipe 29b. The lye level L<2> in the evaporation tank is kept constant by regulating the flow through the pipe 29b by means of devices such as the valve 29c. Thus, if more liquid is evaporated in tank 13a, more steam is removed in pipe 30b and less lye remains in the tank, which tends to lower the level IA Lowering the level L<2> reduces the flow 'through the pipe 29b which thus lowers the level L' in the boiler as less lye enters the boiler.

In the above manner, the amount of evaporation in tank 13a regulates the chemical concentration of the fortified lye and regulation of the flow through pipe 29b regulates the amount of lye circulated to the digester.

The present system for temperature regulation by evaporating a varying amount of water from circulated lye can be used in general where it is desirable to keep the lye-wood ratio at a value that is lower than that which would otherwise prevail as a result of water entering in the system with the chip, the steam and supplementary lye. Thus, this method can be used regardless of the construction of the boiler or the direction of the chip flow (that is, upside down or at an angle) and the steam generated by evaporation can be used for secondary purposes in a plant.

It is; thus a system has been found for regulating the water balance by providing a varying amount of evaporation in the impregnation pipe. This allows a constant lye-chip ratio as the chip passes through a constant environment for regulated chemical concentration at a regulated temperature, and for a regulated time.

Cooking and washing.

Referring to fig. 1, the tile sinks after being impregnated through a rotary valve 14 into the top of the cooking tube 15 as described above. The boiler 15 is of similar construction to the impregnation container 13 and is provided with a conveyor which is schematically indicated at 31. Just as in the impregnation container, 13 the chip is guided downwards along the top of the pipe 15. High pressure steam is injected into the boiler pipe through a steam pipe 16 to keep the specified boiling temperature. The cooking time is regulated by the speed of the conveyor 31, which can be varied to achieve a correct cooking of the chips.

With a dejte boiling system where the boiling is carried out in a separate part in the steam phase where the tile is continuously fed through in an inclined position, black liquor that is strained from the tile during boiling can easily be strained from the boiling pipe to the black liquor container 42. A high percentage (approximately 50%) of the total water content that entering the digester is removed in this way and at a solids content of approximately 30%. With this system, the capacity of the washers can thus be reduced as a large amount of the black liquor is removed in the boiler tube 15.

If desired, a sump 17 of lye can also be kept in the boiler 15 with regulated draining. It is also possible to circulate part of the black liquor from the boiler 15 through a heat exchanger and evaporate this to give steam in a similar way to that used in the impregnation container. By thus evaporating to produce steam, the amount of high-pressure steam injected for boiling can be reduced and the solids content of black liquor in the boiler 15 will be increased. This lye is drained off through pipe 32 and is still at a higher concentration than when direct steaming is used.

Cooked chips exit at the top of the cooking tube 15 and pass through a rotating valve 18 to the bottom of a diffusion washer schematically shown at 19. In the washer 19 which is operated at a pressure approaching the pressure of the boiler, the chips are fed by means of the conveyor 20 in countercurrent just opposite the washing liquid which enters through the pipe 37 and can then be fed to the intake in a pressure washer 21 where another washing step is carried out. The thus-cooked and washed mass is removed from the system via the pipe 22.

The diffusion washer 19 essentially consists of a tube arranged at approximately the same angle to the horizontal as the impregnation tube or the cooking tube and has a screw conveyor 20 to carry the material upwards through the washer as shown. Washing liquid is introduced towards the top of the washer by means of the pipe 37 to go in counter-current just opposite the material which is carried from the conveyor 20.

Black liquor is removed through the strainer 19 near the top of the vertical part 19a in the washer 19 and is led away via the pipe 41.

By moving the boiled mass and the washing water in countercurrent to the washing water that enters the washer and goes downwards and then upwards towards this end of the washing cycle, black liquor at a relatively high concentration and temperature is available to the pipe 41. The boiled mass is cooled and washed kes in the washing machine.l9.

In the embodiment shown, washing water from the pipe 38 enters the pressure washer 21 with the cooked chips through the pipe 39. The lye from the pressure washer 21 is led via the pipe 37 to the upper part of the diffusion washer 19 to go in countercurrent to the cooked mass. It can be emphasized that all the washing liquid required is available as evaporator condensate obtained from a subsequent stage in the system.

Another form of washing system is shown in fig. 3. With this system, a sump of lye 69 is obtained in the pipe 68 in the pipe that exits the boiler. The mass from the cooking pipe passes through the sump 69 and through the valve 18 or similar into the pipe 61. The pipe 61 leads the mass to the tank 62 from where it goes via the pipe 63 to a regular rotating drum washer 64 whose contents are at atmospheric pressure. Part of the lye is passed with the boiled chips through the valve 18 and is removed by the washer 64. Lye returned to the pipe 68 and in excess of what passes through the valve 18 rises in the pipe 68 in countercurrent to the boiled chips and thus increases its concentration to a certain extent while its temperature is simultaneously increased to approx. 150° C to obtain a hot liquid sump. This heated lye is strained from the sump 69 through the strainer 67 and the tube 66. Lye in the tube 66 is combined while it is still at boiling pressure with the lye that is strained through the tube 32 from the boiling tube 13 and sent to the heat exchanger 42 through the tube 33 and thus preheats the incoming boiling reaction agent which passes via the pipe 28 to the impregnation system.

Using methods as shown in fig. 1 and 3, a major part of the heat in the cooked chips is extracted by the fact that, while they are at boiling pressure, they come into contact with lye at a lower temperature, obtained from the subsequent washing step. This allows the discharge of chips from the pressure zone at a temperature that will prevent the development and rapid expansion of steam into the chips and thus reduces the destruction that usually occurs when blowing pulp. At the same time, the temperature in the lye obtained from secondary washing of the boiled chips is increased to a value that approaches the temperature for boiling and thus allows its use for preheating fresh lye on the way to the •impregnation pipe and thus reduces the steam requirement for the system. The system shown in fig. 1 provides more effective primary washing than what is shown in fig. 3 while the system shown in fig. 3 is simpler and involves less capital investment than what is shown in fig. 1.

Other forms of washers can be used, but the described systems are preferred.

Heat recovery system.

One continuous source of steam from the impregnating tube 13 is readily available for any useful purpose. In the embodiment shown, steam is led from the impregnation tube 13 via the tube 30 to a first evaporator 44 for con-

centering of black liquor. The steam supply to the evaporator 44 is supplied if necessary with steam from the pipe 30A which can be obtained from the boiler. Steam from the evaporator 44 is led to another evaporator 45 through a pipe 46, while condensate from the first evaporator 44 is led to an evaporator tank 48 via the pipe 47. To increase the steam supply to the second evaporator 45, steam from the evaporator tank 48 is led through the pipe 49 to the evaporator 45 .

Steam from the second evaporator 45 provides a source of low pressure steam (about 0.4 kg per cm<2> for a system in which the pressure of secondary steam from the impregnator tube is 2.5 kg per cm<2>) in the tube 50. Condensate from the evaporator is extracted as hot water and is carried away via pipe 51. The hot water extracted in this way is supplied to condensate from the evaporator tank 48 via pipe 52 to provide sufficient hot water for supply for all needs of the washing system as well as to provide additional consumption by causticization.

In the case of the washing system shown in fig. 1 provides black liquor at a high temperature (in excess of 100° C and probably between approx. 143 and 160° C) a suitable heat source where heat can be easily extracted in pipe 41. Black liquor at boiling temperature and pressure desilted from pipe 15 into pipe 32 is added to the lye in pipe 41 as supply to the heat source. In the embodiment according to fig. 1, black liquor is led from pipes 32 and 41 via pipe 33 while it is still held at elevated pressure to a heat exchanger 42 which serves to heat supplementary liquor which is fed to the system. From the heat exchanger 42, the black liquor goes to an evaporator tank 43. Steam from the evaporator tank 43 is available for further use, for example in the evaporator 11.

Black liquor from the evaporation tank 43 is fed via the pipe 53, oxidation tower 54 if required and pipe 55 into the evaporator 45 and from the evaporator 45 to the evaporator 44 through the pipe 56. In some methods an oxidation tower can be omitted and the residual liquor is fed directly from the heat exchanger to the evaporator 45 Evaporated liquor from the evaporator 44 enters the evaporator tank 58 through the pipe 57 and steam produced in the tank 58 is added to low-pressure steam in the pipe 50 through the pipe 59. Concentrated black liquor from tank 58 is led by means of the pipe 60 to a suitable recovery system for burning chemicals and heat recovery.

As the impregnation system according to the present invention allows a lower liquid-chip ratio, the black liquor concentration from the washers is higher than with normal systems. The type of washers used will also contribute to the high concentration'. The high black liquor concentration reduces the amount of evaporation required and therefore the use of two effects of evaporation is possible (in addition to evaporation in the impregnation tube). In normal systems, five effects are usually required with accompanying higher heat requirements. The two-stage system of the present invention provides a source of low-pressure steam (0.4 kg per cm<2> for entering steam pressure of 2.4 kg per cm<2>) while a five-stage system provides no steam for recovery (the steam from the end of a five stage system would be at negative pressure about 700 mm's pressure).

A further source of heat economy is available from the combustion of chemicals as there is less inorganic matter present and therefore less heat is carried away.

It is clear that with the system according to the present invention it is possible to reduce heat demand.

The following is a specific example of a heat balance that can be obtained with the system according to the present invention illustrated in fig. 1 for a power process. All weight specifications, if nothing else is stated, are stated in kg per tonne pulp unbleached and 90% dry basis.

Chips are steamed in the evaporator 11, using 317 kg of steam for 3100 kg of moist wood containing 1270 kg of H>0 as moisture and this wood together with 1550 kg of water at 100° C goes to the impregnation container 13. A temperature of 140° C is maintained in the impregnation container 13 with recycling of lye that is heated to a temperature of 155° C by means of the heat exchanger 26. This recirculating lye has a ratio of 15 to 1 in the present example (15 parts recycling lye to one part complementary lye) as the lye when it enters the impregnation device it has a concentration of 33.7 grams per liter effective NaaO and removed from there at 30.5 grams per liter with added lye with a concentration of 90 grams per liter of effective Na20 at a temperature of 140° C.

Steam at a pressure of 2.4 kg per cm<2> and with 1090 kg per tons of chips generated in the impregnator as a result of overheating of lye as previously described is available from the impregnation pipe in the pipe 30 for repeated use.

The temperature of the incoming recirculating liquid is regulated to provide the exact amount of evaporation required to maintain a proper liquid level in the impregnation vessel.

Impregnated chips are fed through the valve 18 to the cooking tube which in this example holds

des at a temperature of 185° C using 515 kg of high-pressure steam. 1780 kg of black liquor at 185° C is drained from the boiler 15 via the pipe 32.

835 kg pulp and 1780 kg lye pass through the diffusion washer in countercurrent to the wash water. This mass and the black liquor that remains after the diffusion washing goes to a pressure washer. The resulting pulp removed from the system by means of pipe 22 contains 835 kg of pulp and about 32 kg of solids in black liquor and 800 kg of water and is at a temperature of 93° C. The black liquor leaving the washer 19 through pipe 41 is at a temperature of 147°C.

Steam In pipe 30, 540 kg of steam is added at a pressure of 2.4 kg per cm<2> from the pipe 30A from a boiler or other suitable source to supply a total amount of steam of 1620 kg to the first evaporator 44.

From the evaporator 44 comes 1510 kg of steam evaporated from the black liquor at 121° C and 1620 kg of condensate. The steam from the evaporator 44 is fed to the evaporator 45 while the condensate from the evaporator 44 is evaporated in the tank 48 to produce 50 kg of steam at 121° C and 1530 kg of condensate. The steam from the evaporation tank 48 is also fed to the evaporator.

The evaporator 45 produces 1170 kg of steam at 180° C when evaporating black liquor and 1555 kg of condensate. This condensate is added to it from the evaporation tank 48 with a total of 30 kg of condensate at 121° C which is sent to the heat exchanger by incoming cold water and then 2640 kg of this condensate is fed at 93° C to the washer via pipe 38, the rest of the condensate goes to the recycling system for use in dissolving the smelt.

As indicated above, 2,640 kg of condensate is supplied to the washing system from which 4,030 kg of lye at 147° C is removed in pipe 41. 2,230 kg of black liquor that is strained from the boiled chips is available at 185° C in pipe 32 and this lye is combined with the lye in the pipe 41 and gives a total amount of 6270 kg of black liquor at 160° C which is fed to the heat exchanger 42 via pipe 33. The temperature in the black liquor leaving the heat exchanger is reduced to 141° C and the black liquor is then evaporated in tank 43 and gives 390 kg of steam, part of which is fed to the evaporator. 5870 kg of black liquor from the evaporator tank 43 passes through an oxidation tower 54 from which 5300 kg of liquor at 73° C is fed to the series of evaporators 45 and 44 and finally to the evaporation tank 58. In the evaporation tank 58 steam is produced in an amount of 100 kg, which is added to the treatment steam from the other, 45 evaporates in a total amount of 1235 kg treatment steam at 0.4 kg per cm<2>. 2590 kg of black liquor is fed from the evaporation tank 58 via the pipe 60 to the standard recycling system.

The treatment steam used in the heat exchanger 26 to heat the recirculating lye for the impregnation resulted in 1180 kg of condensate.

Table 1 below is a comparison of the heat economy for the continuous cooking system described above with a batch system. Unbleached spruce chips were used in this comparison.

From the above table, it is clear that the steam requirements with the present invention are significantly reduced compared to a normal system. This reduction in heat requirements is mainly due to the smaller amount of chemicals carried by the tile. As previously pointed out, more heat is available from the regeneration furnace as there is less inorganic material in the smelt that can carry away heat. Less heat is also required for evaporation as the system according to the present invention can be operated with only two stages of evaporation and some heat can be recovered from the evaporation system.

Example 1 in the following demonstrates an embodiment of the continuous cooking system where a power process is used and includes a description of the mass produced. In this system, the described washing systems are not used.

Example 1:

Spruce chips containing 40.5% moisture was fed at a rate of 1.1 kg per minute dry chips to the evaporation vessel in an experimental plant with a continuous boiler similar to that shown in fig. 1. Steam was supplied to the pipe at a rate sufficient to maintain a pressure of 0.7 kg per cm<2>. The condensate formed was filtered from the bottom of the pipe. The conveyor speed was set so that the residence time for the chips in the evaporation vessel was 5 minutes.

The vaporized chips were introduced into an impregnation container in which a liquid level was maintained up to the level of the chip intake. The tile was passed through this vessel by means of a conveyor whose speed was kept so that it gave a residence time of 30 minutes for the impregnation. The concentration of the effective alkali in the feed liquor was kept at 32.6 grams per liters by adding fresh lye to the lye that was recycled through the circuit-flow pipe. To maintain this concentration, a liquid flow of 2 liters per minute at a concentration of 84 grams per litres. The fresh lye was produced with a sulphidity of 30%, while the lye in the impregnation tube reached equilibrium with a sulphidity of 45%. The lye in this tube was kept at a temperature of 136° C at a pressure of 2.1 kg per cm<2>. The lye was superheated to a temperature of 145° C in a circulating heat exchanger and allowed to evaporate to 136° C so that a water balance was maintained in the impregnation tube.

After impregnation, the chip was introduced into the cooking vessel which was kept at 10 kg per cm<2>'s pressure when supplying direct steam. The residence time for the tile in the cooking zone was 30 minutes. Condensate and lye which was drained from the tile as the boiling progressed was removed from the bottom of the boiler as the boiling was carried out essentially in the vapor phase. At the end of the boiling period, the chip was removed from the digester through a rotary valve and allowed to vaporize to atmospheric pressure.

A sample taken from the mass showed the following characteristics:

Chlorine number 4.5

C.E.D. viscosity 15.6

G. E. brightness 29.7

Visible debris (percent of mass) 1.76.

The pulp was beaten in the Valley Beater and showed the following strength characteristics:

In a boil which was carried out in a similar way, but using lye with 100% sulphidity, pulp samples were obtained from the digester by & blowing in the usual way and also by cooling boiled chips to 100° C before draining. The following table illustrates a comparison of cold discharge with hot.

From the above comparison, it is clear that the characteristic for cold exhausted pulp is better than from hot blown pulp. With a cold exhaust system according to the present invention, fiber destruction and reduction in strength are reduced compared to hot-blown exhaust.

A higher percentage of heat from the system is in the form of high-pressure steam than in a normal batch system. A smaller total heat exchange area is also required for boiling and evaporation than is the case for normal batch operation and a high percentage of secondary heat from the described system can be extracted.

The impregnator and the method for impregnation are described in connection with cellulose cooking systems. This method and apparatus can also be used in other environments for the impregnation of materials that carry varying amounts of water and where the impregnation agent is contained in water or another evaporable liquid.

In some extreme cases it is necessary

no evaporation takes place in the impregnation tube. If for example not

sufficient water was to enter the impregnation pipe, the level of the liquid sump had to be raised

is regulated by spraying water into the impregnation pipe. This would probably just

happen if the impregnator was fed with dry

chips such as from kiln-dried wood. Below these

circumstances, regulated amounts of water are added to the impregnator according to level L

to maintain the level inside the pipe.

The liquor recycled through the heat exchanger 26 would be heated to maintain the temperature of the liquor, but due to

of too little water would little or no steam

removed from the impregnator. The employee

water quantity can be regulated by testing

the water level and regulate added water to

maintaining level L essentially constant. Lye concentration and temperature,

the pressure in the impregnation tube and the time that

the tile uses to pass through the pipe

is naturally regulated to achieve a correct

impregnation of the tile.

Although the description mainly refers to chips, others may contain cellulose

raw materials are processed in the system.

Claims (4)

1. Procedure for continuous boiling of cellulosic raw materials, where the material is subjected to impregnation with an aqueous solution of cooking chemicals in an impregnation system to impregnate the material with cooking chemicals in a specific weight ratio, and in which water enters the impregnation system as moisture in the raw material and as water in the white liquor and leaves the system with it impregnated raw material, characterized in that a water balance in the impregnation system is maintained by: controllable evaporation of a quantity of water corresponding substantially to the difference between the quantity of water entering the impregnation system and the quantity of water leaving the system with the impregnated raw material, steam formed during the evaporation removed from the system in order to prevent the accumulation of water in the system. 2. Method according to claim 1, where a liquid level is maintained in an impregnation zone, and the cellulose raw material is continuously fed to this liquid, and heated impregnation liquid is introduced into the impregnation zone, characterized in that the introduced heated impregnation liquid has a temperature that is higher than the temperature of the liquid in the impregnation container and the temperature of the heated impregnation liquid and the pressure in the impregnation container is regulated so that part of it evaporates , the quantity of the evaporated impregnation liquid and added liquid being such that the level in the impregnation container is kept essentially constant. 3. Method as stated in claims 1 and 2, where liquid is continuously removed from the impregnation container, characterized in that a constant level is kept in the container by indirect heating of the liquid that comes out of the container, evaporation of part of the liquid from the container in a separate tank with constant pressure, during the formation of steam that is extracted from the system, and that the rest of the liquid is sent back to the impregnation container in order to keep the system in water balance. 4. Apparatus for carrying out the method as stated in claim 3, characterized by the combination of a boiler, an evaporator tank, devices for indirect heating, where the boiler has a cooking zone and an impregnation zone filled with impregnation liquid, devices for transporting impregnation liquid from the boiler to the indirect heating device and further to the evaporator tank, devices for transferring impregnation liquid from the evaporator tank to the boiler, devices for regulating the flow of liquid from the evaporator tank to the boiler to keep the level of impregnation liquid constant in the evaporator tank, as well as devices for regulating the temperature of the incoming impregnation liquid to the evaporator tank in accordance with the level in the impregnation container, so that a regulated amount of water is evaporated in the evaporator to keep said level constant, as well as devices for extracting utility steam from the evaporator tank.